Another way a star finishes its life is by becoming a black hole. The "event horizon" of a black hole star is a location, spherical in shape, at which the escape speed is equal to the speed of light (c = 3.00 x 10^8 m/s). What is the radius of such a sphere for a black hole of mass 1.9 x 10 kg ? [Note. This is what we generally refer to as the "size" of a black hole. Be sure to include a complete derivation, starting from basic principles.]